In China, there are many traditional(传统的)holidays. The most important one is the Spring Festival. People like it very much. On the Spring Festival, people don't work. Children don't go to school. People clean their rooms. Mothers make dumplings. Kids can get lucky money. Besides this festival, the Dragon Boat Festival and the Mid­Autumn Festival are also popular. On the Dragon Boat Festival, people eat zongzi and have dragon boat races. On the Mid­Autumn Festival, people eat moon cakes and watch the moon. How happy they are!

如图，△ABC中，∠C=90°，AC=3，AB=5，D为BC边的中点，以AD上一点O为圆心的⊙O和AB、BC均相切，则⊙O的半径为____．